I just wanted to get some advice on when the peak fall colours are usually at Zion or The Great Smokies.

Looks like no one is going to chip in with reco's, so I'll start the ball rolling with my experiences. I've never been there earlier than early November, and things were past their peak then. Still lots of leaves on the ground, but the trees were getting pretty bare. If I was to go back in the fall for colors, I'm guessing mid-October, but I'll defer to any more specific answers you get.